A particular type of modems for full-duplex data transmission over the PSTN has been defined in the CCITT Blue Book, published by the CCITT in Geneva, 1989, in Volume VIII, Fasc. VIII.1. The respective Recommendation V.32 is entitled "A family of 2-wire, duplex modems operating at data signalling rates of up to 9600 bit/s for use on the general switched telephone network and on leased telephone-type circuits". In this recommendation, a start-up procedure is described for exchanging signals between a call mode modem and an answer mode modem in order to train the echo cancellers and equalizers of these modems. This procedure is based on sending essential parts of the training signals in a half-duplex manner. The start-up time required for executing this procedure is in the range of 4 to 8 seconds, which is considered too long for many uses. The follow-on Recommentation V.32 bis, published by the CCITT in 1991, describes essentially the same start-up procedure, but allows for higher data rates than V.32.
A method suited for training echo cancellers in full-duplex modems faster than in V.32 modems is described in a publication by J. M. Cioffi: "A Fast Echo Canceller Initialization Method for the CCITT V.32 Modem", IEEE Transactions on Communications, Vol. 38, No. 5, May 1990, pp. 629-638.
Some new aspects of full-duplex modem initialization are presented in a publication by X. Chen at al. entitled "A Full-Duplex Fast Training Algorithm for Simultaneously Estimating Echo and Channel Response", Proceedings SUPERCOMM/ICC '92, Paper 351.1, pp. 1503-1507. A technique for simultaneously estimating the responses of near-end and far-end echo paths and the signal path from the remote modem to the local modem is described. These responses are then used to adjust the echo cancellers and the equalizer. The authors propose to send for this purpose known periodic training sequences. One reason for this is the difficulty for the receiving modem to determine the beginning of a non-periodic training sequence sent by the remote modem. This is one of the problems solved by the present invention.